This application claims the Paris convention priority of Japanese Patent Application No. 11-294477/1999 filed on Oct. 15, 1999, which is incorporated herein by reference.
The present invention relates to a lithium secondary battery, and more particularly, it relates to improvement of a negative electrode active material for the purpose of improving the charge-discharge cycle performance of a lithium secondary battery.
In a lithium secondary battery, the battery characteristics such as the charge-discharge voltage, the charge-discharge cycle performance and the storage characteristic are greatly varied depending upon active materials used in its positive and negative electrodes.
As an active material of the negative electrode, metallic lithium is well known. In a lithium secondary battery using metallic lithium, lithium is deposited from the electrolyte during charge and is dissolved into the electrolyte during discharge. Although the lithium secondary battery can attain a high energy density per unit weight and unit volume by using metallic lithium, there arises a problem that lithium is deposited in the form of dendrite on the negative electrode during charge, resulting in causing internal short-circuit.
In order to overcome this problem, metallic aluminum has been proposed as a negative electrode active material usable instead of metallic lithium (Journal of Power Sources, 12 (1984), pp. 145-153). In a lithium secondary battery using metallic aluminum, lithium is inserted into the metallic aluminum during charge and is released from the metallic aluminum during discharge. Differently from the use of metallic lithium where lithium is dissolved and deposited, the metallic aluminum merely serves as a host for absorbing and discharging lithium. Accordingly, there is no fear of the deposition, during charge, of dendritic lithium causing internal short-circuit.
Metallic aluminum, however, largely expands by absorbing lithium and largely shrinks by discharging lithium. In other words, the volume is largely and repeatedly changed during charge-discharge cycles. As a result, the metallic aluminum is changed into a fine powder and its current collecting property is degraded, resulting in degrading the charge-discharge cycle performance. This is the reason why a lithium secondary battery using metallic aluminum as the negative electrode active material is practically used as a backup battery for a memory merely at a small depth of charge-discharge of approximately several percentages in spite of its theoretical capacity (i.e., 992 mAh/g) much larger than that of graphite (i.e., 372 mAh/g).
Accordingly, an object of the invention is providing a lithium secondary battery exhibiting good charge-discharge cycle performance by using a negative electrode active material with small volume change during charge and discharge.
The lithium secondary battery of this invention (present battery) comprises a positive electrode; a negative electrode; and a nonaqueous electrolyte, and the negative electrode uses an active material formed from an alloy including an A phase of a first intermetallic compound (A), and a B phase of a second intermetallic compound (B) having the same constituent elements as and a different composition from the first intermetallic compound (A) and/or a C phase consisting of one of the constituent elements of the first intermetallic compound (A), and at least one of the A phase, the B phase and the C phase is capable of electrochemically absorbing and discharging lithium ions.
As a result, the lithium secondary battery of this invention can exhibit good charge-discharge cycle performance.